Rabbits were fed 100 g/day of a standard ration containing 1% chloesterol.One group of 5 was supplemented with 20 mg/kg/day of adenosine and 8.5 mg/day vitamin B2 tetrabutyrate.Another group was supplemented with 8.5 mg/day of vitamin B2 tetrabutyrate, and a third group was given only the cholesterol-fortified ration.After 6-weeks feeding the change in vitamin B2 quantity in liver, kidney, and heart was measured and total cholesterol, phospholipid triglyceride, lipid peroxide, and protein were determined in the blood.Both adenosine and vitamin B2-tetrabutyrate treatment significantly raised vitamin B2 levels over controls, the joint administration giving higher levels than with vitamin B2 tetrabutyrate alone.